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+# LANGUAGE pragma Haskell extensions
+
+List: https://downloads.haskell.org/~ghc/7.0.3/docs/html/libraries/Cabal-1.10.1.0/Language-Haskell-Extension.html
+
+## Using
+
+Put as *first* line of file:
+
+    {-# LANGUAGE Extension, ... -#}
+
+## FlexibleContexts
+
+Relax some restrictions on the form of the context of a type signature.
+
+Normally typeclass constraints have to have type variable arguments.
+
+    modify :: MonadState s m => (s -> s) -> m ()
+
+With FlexibleContexts, can be any type (?)
+
+    modifyFst :: MonadState (a, b) m => (a -> a) -> m ()